Fluorescent lamps are highly useful as light sources for backlighting flat panel liquid crystal display because of their high light emission levels, their high efficiency and because the spectral characteristics of the emitted light can be readily controlled. In the past, fluorescent light sources for flat panel liquid crystal displays typically consisted of parallel arrays of elongated fluorescent lamps mounted on a support element and positioned behind the display.
In backlighting flat panel liquid crystal displays, the active area of the display must be uniformly illuminated by the lighting source. In addition, in many applications as for example in aircraft cockpit displays where space is at a premium, the display panel must be as small as practical; i.e., the non-active border of the panel should be minimized. Fluorescent lamps with all their advantages of high light output and high efficiencies, etc., do have one shortcoming with respect to such applications because the light output in the vicinity of the lamp electrodes is low. That is, the relatively dark area between the tip of the lamp and the point where phosphor excitation takes place may be more than an inch long. In a 4.times.4 or 6.times.6 inch display this represents a substantial area in which a light output is much lower than the output from the main portion of the light source, thus severely eroding the uniformity of the light illuminating the liquid crystal display.
In attempts to overcome the non-uniformity display problem, displays have been made where the area of low light emission adjacent to the electrodes are located in the border or inactive area of liquid crystal display. This, however, increases the border area and the overall size of the flat panel display for a given active display area. This, as pointed out above, is undesirable in those applications where space is at a premimum and where the flat panel display must have minimal border area.
Applicant has found that a fluorescent light source for liquid crystal displays with high uniformity, may be realized by providing a fluorescent lamp having the portions containing the electrodes bent out of the plane of the principal light emitting portion of the fluorescent lamp. As a result, the relatively dark portion of the lamp in the vicinity of the electrodes is not in the light emitting plane which illuminates the liquid crystal display. Furthermore, by bending the electrode containing portions of the lamp out of the plane of light emission, the non-active border area of the liquid crystal display is minimized.
In addition to bending the electrode containing lamp portions out of the plane of light emission, it is also desirable to have the main portion of the fluorscent lamp bent back on itself in a u-shaped configuration to increase the length of light-emitting portion of the lamp for each pair of electrodes. This improves efficiency of the flurorescent lamp by eliminating the I.sup.2 r losses associated with the extra pair of electrodes needed if simply two elongated fluoresent lamps are used. That is, for a 4.times.4 liquid crystal display, for example, a u-shaped fluorescent having two legs each four (4) inches long gives a total light emitting length of eight (8) inches. If two linear four (4) inch fluorscent tubes are used, four electrodes are necessary thereby doubling the I.sup.2 r losses without increasing the light emission. Thus, a combination of a three-bend fluorescent light source, with two of the bends bringing the electrodes out of the plane of light emission and one bend to form the u-shape and other bend-back configuration, provides substantial advantages in terms of efficiency and uniformity of lighting.
It is therefore a principal objective of the instant invention to provide a fluorescent light source for a liquid crystal display which provides uniform illumination of the display while minimizing the overall size thereof.
Yet another objective of the invention is to provide a light source for a Liquid Crystal Display in which the electrodes of the fluorescent light are out of the plane of the light-emitting portion of the lamp.
Still another objective of the invention is to provide a fluorescent illumination source for liquid crystal display in which the number of fluorescent lamp electrodes are minimized.
Still other objectives and advantages of the invention will become apparent as the description thereof proceeds.